iStorage DiskAshur2 HDD (1TB)

LaCie Rugged Thunderbolt USB-C Mobile Drive

First the good news: Research analysts at IDC estimate that we store less than 1 percent of the data that we create. Now the bad news: IDC says the world will be creating 163 zettabytes—that's 163 billion terabytes—of digital data per year by 2025, as the Internet of Things overwhelms the globe with sensors and signals.

While the amount of storage being shipped can't begin to keep up with the flood of data, there's still plenty of information that needs to be stored, starting with all of yours. In this age of high-definition and 4K video (a zettabyte, by the way, is 36 million years of HD video), monstrously big RAW photo files, and endless PowerPoint pitches, almost no computer's internal hard drive or solid-state drive (SSD) will have enough headroom to last indefinitely. Chances are, there will come a day when you need more storage space.

Meanwhile, the day has already come when you need to think about the safety of the data on that drive—it was day one. If you're not making frequent backups, you're asking for terabyte-size headaches. Free and low-cost cloud services such as Dropbox and Microsoft OneDrive, or old-fashioned recordable DVDs, can hold a few gigabytes; flash drives can hold a few dozen.

External SSDs are fast, sturdy, and first-class solutions for small to medium storage and backup needs; that's why we published a shopper's guide to them. But their price per gigabyte remains relatively steep. If you're on a budget, or seeking more than a terabyte or two of convenient storage, the smart solution for both adding storage and doing daily backups remains an external hard drive.

Seagate Personal Cloud (3TB)

The best external hard drives offer anywhere from 1TB to 8TB (in a few cases, even more) of handy, simple storage while taking just a bit of desktop space. And they're affordable, with prices well under $100 for many of the 1TB and 2TB models on the market.

Here's what to look for as you shop.

Buying an External Hard Drive: The Basics

SIZE AND SHAPE. As you shop for the best external hard drive for your needs, you should consider a drive's physical size and power requirements. External drives fall into two main size classes: shirt- or jacket-pocket-sized portable models (based on 2.5-inch laptop hard drives) and much larger, desktop-only units (based on the bulkier 3.5-inch hard drives used in desktop PCs).

Most 2.5-inch drives nowadays are bus-powered, meaning that they draw the power they need from the same cable connection they use for data transfer, with no AC adapter required. Desktop drives, meanwhile, will have their own power adapter. That, obviously, affects whether your drive is a stay-at-home or can travel with you and work away from a power plug.

That said, know that the larger-bodied desktop-style drives will have the maximum possible capacities, if getting the most storage space possible is your main concern. Most of the most spacious single-drive desktop models top out at 8TB and cost roughly $200 to $300. If you see anything higher-capacity, at least at this writing it was most likely using two or more drive mechanisms inside the box. Note that some 8TB boxes, too, use two 4TB drives inside for speed or redundancy.

WD My Book (8TB)

Most bus-powered portable drives max out at 2TB or 4TB. A few of the latter use two drives in a RAID arrangement inside their shells.

INTERFACE CHOICES. Beyond capacity and form factor, your next most important choice is a drive's interface—the connection to your computer. The most popular at this writing is USB 3.0—blazingly faster than USB 2.0, even if its real-world speed advantage isn't the 10 times that some vendors advertise based on theoretical bandwidth.

It's easy to get confused by different flavors of USB. USB 3.0, which uses the rectangular Type-A connector usually colored blue or marked with SS for SuperSpeed, has the same throughput or speed ceiling as USB 3.1 Gen 1. The latter comes with either a Type-A connector or the smaller, oval-shaped Type-C connector that, conveniently, has no right side up or upside down. Many drives come with both Type-A and Type-C cables to fit older or newer PCs.

USB 3.1 Gen 2 and Thunderbolt 3, which use the Type-C connector, are faster still—but their extra speed is largely wasted, just unused bandwidth, for conventional hard drives as opposed to SSDs. (See our primer on USB and Thunderbolt technologies.) Suffice it to say that USB 3.0 or 3.1 Gen 1 will exist, and be good matches for external hard drives, for years to come.

LaCie Rugged Thunderbolt USB-C Mobile Drive

Some Mac computers use an older version of Thunderbolt, and some aging Macs use a vanishingly old interface called FireWire (a.k.a. IEEE 1394) which you might see on the occasional used or closeout drive. Ditto for USB 2.0, which you want to avoid even if you have an old PC—your USB 3.0/3.1 drive will work (albeit at reduced speed) with that PC, and your next system will support the drive natively.

For a different kind of interface altogether, a few battery-powered hard drives rely on Wi-Fi. Drives like these serve as a combination of a USB external drive and a Wi-Fi access point, and they allow convenient (if relatively slow) over-the-air file transfers among a group of friends or coworkers with laptops and tablets. They're a whole other class of external drive, so check out our wireless drive roundup for more info.

Instead of software, the iStorage DiskAshur 2 offers keypad password protection and even a self-destruct function.

SOFTWARE AND THE CLOUD. The best external hard drives also provide bundled software for automated, scheduled backup, whether of just your data files or the entire drive. Look, too, for a utility that offers password protection or full or partial drive encryption, which can keep the drive or selected folders from prying eyes should you lose it.

Most drives are ready to use out of the box, but some feature additional setup utilities such as LaCie's, which lets you drag a slider to specify what percentage of the drive you want to be Mac-only and how much you want shareable between PCs and Macs. And some take advantage of the cloud, such as the Seagate Duet, which backs up anything you save on the drive to a first-year-free account with Amazon's online storage service. By contrast, the Seagate Personal Cloud is less a cloud device than a home or family network-attached storage (NAS) drive or central data depository.

Seagate Backup Plus Hub (8TB)

DURABILITY. Finally, you may want to consider a drive's ruggedness if you'll be toting it around in rough environments—or if you need it to be fire- and crush-proof, even if it never leaves your office. At the basic end of the scale, this can be a simple rubber bumper or case to protect a portable drive from the jolts and jostles of travel (though spinning platter drives, as we said, remain more fragile than SSDs or flash storage). At the far opposite extreme is the ioSafe Solo G3, a disaster-proof desktop drive warranted to survive 1,500 degrees Fahrenheit during a fire or 72 hours under 10 feet of water.

Our Verdict: The Backup Plus Hub is as cavernous as it is affordable, and its front USB ports are a nice "where-have-you-been?" bonus. The software could use a few tweaks, but it worked well overall. Read Our Review

Our Verdict: This small, budget-minded NAS device provides good bang for the buck, packing ease of use and comprehensive media-streaming and backup apps. Just recognize the no-redundancy risk posed by any single-drive NAS. Read Our Review